Power drilling systems include a positive feed spindle which is equipped at its leading end to support a working tool such as a drill bit, counter sink, or reamer. The spindle is mounted in a drill head housing for both rotational and longitudinal movement and is rotated and advanced or retracted by differential gear systems commonly referred to respectively as spindle drive and spindle feed gear trains. It is a conventional practice to provide the drive spindle with positive action depth sensing means which can be adjusted longitudinally along the length of the spindle. The depth sensing means can initiate one or more of spindle functions upon contact to the depth sensing means with a positive stop shoulder in the spindle housing. For example upon contact of the depth sensing means with the stop shoulder spindle advance or rotation may be stopped or the spindle may be automatically retracted. In addition, contact of the depth sensing means with the stop shoulder may cause the spindle to act by the function of a limited slip coupling in the spindle drive in a dwell mode in which the spindle is rotated without advance in order to ream the surface of a previously drilled hole, as disclosed in U.S. Pat. No. 4,557,646 to Biek.
Various systems are known for adjustably positioning such sensing means along the length of the spindle in order to designate a point of spindle advance at which a desired drilling or control function is initiated. For example U.S. Pat. No. 3,767,313 to Bohoroquez et al discloses a positive feed drill in which the drill spindle is mounted in a drill head housing equipped with a sensing sleeve mounted for telescopic movement in the tool nose. The sensing sleeve is spring biased to project the sleeve out of the tool nose and into contact with a work piece. The slidable sleeve has a stop shoulder which, when the sleeve is in contact with the workpiece, is at a predetermined distance from the workpiece. The spindle is provided with adjustable stop means which upon contact with the stop shoulder of the depth sensing sleeve, causes the spindle to operate in a dwell mode for a few revolutions and then retract. The stop means includes a cylindrical stop and guide member positioned immediately forward of a locating ring which is keyed to the spindle against rotational movement but slides longitudinally of the spindle. The cylindrical member and locating ring are adjustably secured on the spindle by means of two threaded collars, referred to as front and rear lock nuts which are threadably mounted on the spindle to be moved longitudinally along the spindle and locked in place. The front lock nut is of a sufficiently reduced diameter so that it passes through the stop shoulder which is contacted by the cylindrical member to initiate the dwell and retract function.
An improved form of depth control means, also employed to initiate a dwell and retract function, is disclosed in the aforementioned U.S. Pat. No. 4,557,646 to Biek. The adjustable depth control means in Biek is rotatably mounted on the drive spindle by a thrust bearing assemble and includes a post member which is retained on the spindle by a lock ring and has an exteriorly threaded surface provided with a plurality of longitudinal slots. An adjusting sleeve having a lower surface adapted to engage the stop shoulder of the tool is threaded on the exterior surface of the post member and is provided with a plurality of circumferentially spaced radial holes. These holes are threaded to accept a locking screw. The sleeve is rotated relative to the post member to arrive at a desired depth setting and the locking screw then threaded through one of the radial holes and into the appropriate slot in the post member.
Yet a further improved depth control means is disclosed in U.S. Pat. No. 4,495,681 to Eckman. The Eckman system includes an adjustable depth control means comprising a post member and adjusting sleeve in which the threaded connection between these elements is relatively loose so that the adjusting sleeve can float slightly relative to the post member. In Eckman the adjusting screw is relatively short so that when it extends through the adjusting sleeve and into a post member slot, it stops short of riding on the bottom of the slot.